Numerous pigs, including pipe inspection pigs are in existence and have been used in connection with moving various devices through pipe, including pipeline-cleaning devices and non-destructive inspection of pipelines for gaseous or liquid materials, such as natural gas, liquid hydrocarbons, or water.
Much of the existing underground and undersea pipelines are made out of ferromagnetic steel and for that reason, inspection devices such as eddy current detectors, as may be used in inspection of non-ferromagnetic tubing, such as stainless steel or other types of tubing used in heat exchangers and the like are not particularly useful in connection with underground steel pipelines, such as chemical pipelines, liquid hydrocarbon pipelines, gas pipelines, water or sewage pipes, and the like.
Various methods of detecting flaws or defects from the inside of a pipe or pipeline have been attempted with varying degrees of success. Ferromagnetic induction devices have been used as disclosed in U.S. Pat. No 4,742,298. This invention was directed to determining the presence and the magnitude of surface flaws and to overcoming difficulties encountered in determining the presence and the magnitude of surface flaws in a pipe. The solution proposed was to use a cylindrical primary alternating current coil which is coaxially aligned with the pipe to generate a high frequency AC magnetic field in the pipeline. A multiple cylindrical secondary AC sensing coil was arranged at prescribed intervals in a circumferential direction around the interior of the pipe, each secondary coil having an axis parallel to the axis of the primary coil. The AC voltage sensed at each secondary coil was set to be proportional to the density of a parallel component of magnetic flux caused by the AC magnetic field generator.
Eddy current sensing probes have also been used primarily in connection with non-destructive inspection and testing of relatively thin-walled tubing which was not ferromagnetic material. Such tubing does exist in steam generators and heat exchangers, which have been the primary focus of eddy current probes, such as a single direction rotating head profilometer, as disclosed in U.S. Pat. No. 4,851,773. One embodiment of that device discloses an electromechanical eddy current probe having a rotatable sensing head for sensing the wall thickness and for locating local defects in a tube or conduit. Basically, the mechanical profilometer probe was designed to detect dents in the interior surface of steam generator tubes. The position of the rotating head was varied along the length of the tubing being inspected as the probe was drawn through the tubing with a cable.
Another eddy current probe is disclosed in U.S. Pat. No 4,952,875 in which a plurality of pairs of diametrically opposed sensing coils are shown alternatingly staggered along the longitudinal axis of the test sensor to give complete coverage of the interior pipe surface. Such sensors were further permitted to move in and out to accommodate the size differences or constrictions in the pipeline. However, the sensor probe was intended to move longitudinally through the pipeline.
Also, U.S. Pat. No 5,068,608 discloses multiple coil eddy current probe system and an eddy current probe is disclosed in which a defect was first detected when the probe was positioned adjacent the defect and a series of axially spaced probes were activated to sense and detect the extremities of a crack or other discontinuity. Generally, eddy current probes have not been particularly successful with respect to underground pipelines constructed of steel or other ferromagnetic materials and having pipeline walls with thicknesses substantially greater than the normal eddy current penetration depth. However, one attempt to provide an eddy current probe or ferromagnetic pipeline flaw detection was disclosed in U.S. Pat. No. 4,107,605. There was no indication of the usefulness of such probes in connection with determining longitudinal cracks which were parallel to the direction of movement of the probe assembly.
Popular and useful sensors for ferromagnetic pipeline inspection have been magnetic flux generators and magnetic flux leakage sensors which were positioned circumferentially around an inspection pig which were moved longitudinally through the pipeline. Examples of such sensors were disclosed in U.S. Pat. Nos. 4,105,972, 4,310,796, 4,444,777 and 4,458,601. The operation of such magnetic flux detection probes was described in U.S. Pat. No 4,789,827 in connection with a magnetic flux detection probe in which the sensors were shown intentionally spaced at different radial distances, or spaced different distances from the interior pipe surface, in an effort to obtain greater accuracy with respect to the location of the flaw or defect on the inside or the outside of the pipe wall.
Some attempts have been made to detect defects at different angular orientations in connection with testing and inspecting pipes as they were being manufactured. U.S. Pat. No 3,906,357 disclosed an exterior pipe testing device in which there were two external sensor sections, one having a plurality of fixed sensing shoes circumferentially spaced around the pipe to be inspected which depended upon linear movement of the pipe therethrough for detecting flaws or defects primarily oriented circumferentially around the pipe. A second inspection unit was provided with a pair of opposed magnetic sensing shoes which are rotated rapidly around the outside of the pipe to be inspected in an effort to detect longitudinal cracks which might have otherwise gone unnoticed with the fixed shoe sensing unit. Complex circuitry was used to coordinate the sensor input from each of the sensing units with a rotating magnetic pulse generator geared to the linear motion of the pipe being manufactured. A purpose of this device was to actuate one or more spray cans at the linear and the circumferential position where a manufacturing flaw was detected either by the linear inspection unit or the rotary inspection unit. Application of such a testing device to on-site underground pipelines has not been demonstrated.
Another exterior pipe testing device has been disclosed in U.S. Pat. No 4,439,730, in which pairs of north and south poles of magnets were held adjacent to the exterior wall of a pipe at uniformly spaced apart positions circumferentially around the pipe. The north and south poles were positioned between the north and south poles of longitudinally spaced apart circular magnets around the pipe. The circumferential spaced apart magnets were rotated at a high rate of speed so that orthogonically directed resultant magnetic fields were produced on opposite sides of the pipe between the north and south pole of the rotating magnets. Pairs of flux detectors were interposed on opposite sides of the rotating magnet. The magnets were rotated at a sufficiently high rate of speed relative to the longitudinal motion of the pipe since the flux field interruptions in the same incremental area of the pipe. Again, complex circuitry was required in order to coordinate the sensor input from each of the sensing units because of the high rotational speed (320 revolutions per minute in the example set forth in '730) in order to keep track of the sampled signals from the two overlapping sensors and further, to coordinate them to a longitudinal position along the pipe. At a longitudinal traveling speed of 80 feet per minute as set forth in the example, the device had to make four complete revolutions during every one foot of travel, which was consistent with the sensor field slightly over three inches long, so that 100% of the pipe surface could be covered. Such a device is not considered practical for internal inspection of existing underground pipelines. Potentially, the rate of rotation may not be achievable for internal pipe inspection devices.
Pipeline flaw detectors for use inside of existing pipelines have also provided rotary mechanisms for rotating sensing shoes helically through the pipeline as the detector was moved linearly therealong. One such device was disclosed in U.S. Pat. No. 3,238,448 which, upon detecting a flaw, actuated a strong electromagnet to magnetize the corresponding portion of the pipeline so that the position of the defect could be detected from aboveground with magnetic sensors. This device rotated two opposed search units in a single direction such that only very large flaws could be accurately detected and locating any such detected flaws was dependent upon a second careful searching action for the magnetized pipe section from aboveground.
Another pipeline inspection apparatus was disclosed in U.S. Pat. No 4,072,894 which produces a circumferentially directed magnetic flux field as flux leakage detection sensors were resiliently held against the pipe wall surface and helically moved through the pipe to pass transversely across any longitudinally extending anomalies in the pipe wall. This device produced only a circumferentially directed magnetic flux and produced helical movement of the sensing probes in only one direction.
Another popular and widely used internal magnetic flux gas pipe inspection devices comprised a pipeline pig which had sealing cups around the exterior perimeter to both center the apparatus and to drive it by differential gas pressure along the pipeline. A magnetic flux was generated by multiple circumferentially spaced magnets with north and south poles axially spaced apart and a magnetic flux sensor interposed therebetween. In operation, the pig traveled linearly through the pipeline and sensory input data from each sensor was recorded as a function of distance of travel. When a defect, void, or other anomaly in the pipe was indicated by sensing an interruption of a smooth longitudinal magnetic flux, then such an anomaly was recorded on a graph as a function of time or distance. A major drawback of this device was that the longitudinal, or axially aligned, magnetic flux could not always detect longitudinal voids or defects such as a uniform deterioration along a continuous welded seam of the pipeline. Resolution was determined by the size of the multiple sensor unit. A second set of circumferentially positioned magnetic flux generators and flux leakage sensors could be positioned at a small staggered distance with respect to the first set so that the space between the flux generator and sensor shoes was covered by the second set of sensors. Still, minor disturbances at the start of a longitudinal defect and at a distant end of the longitudinal defect could go unnoticed on a graph.
The best resolution available was approximately limited by the size of the gap between the sensors. Often, one or more of the multiple sensors could fail during a run several miles through a pipeline, which could give an entire line of approximately one to three inches wide in which no discontinuities would be detected along the length of the pipe. In order to reduce some of this risk, the pigs were often rotated at up to about a 1.degree. angle, which amounted to about one revolution per 1,000 linear feet. The magnetic flux was still linearly aligned in the axial direction and the small amount of rotation, if any, was so small that longitudinal voids continue to be substantially undetectable.
Pigs for electronic or for magnetic inspection, as well as pigs for other purposes, such as x-ray inspection, visual inspection, welding, coating and cleaning, have also suffered from deficiencies in the modes of propelling both the pig and the useful devices attached to the pig, through the inside of a pipe. Pigs have been propelled by pipeline flow, but this has not always been adequate. For example, pressure or flow propulsion has not been adequate for empty pipes or for severely leaking pipes.
Inspection pigs have sometimes been propelled with spring-loaded drive wheels, rollingly held against the pipe wall. Spring-loaded idler wheels provided opposed force to hold drive wheels against the pipe. All of the wheels were aligned for rolling parallel to the pipe axis. The traction of the drive wheel in any sludge, which usually accumulated at the bottom of the pipe, was sometimes inadequate.
Some pigs have been drawn through pipe with cables and winches, but this has not been acceptable over very long distances, especially where bends exist in pipeline along the intended path of the pig. The use of cable pulling has required substantial anchoring of the winches. In cases of above-ground pipes, as with pipeline stored in sections, special anchoring of the pipe relative to the winches has also been required.